Cellular communication networks evolve towards higher data rates, together with improved capacity and coverage. In the 3rd Generation Partnership Project (3GPP), the latest technology standard, Long Term Evolution (LTE), is currently being developed and standardized.
LTE uses an access technology based on Orthogonal Frequency Division Multiplexing (OFDM) for the downlink, and Single Carrier Frequency Division Multiplexing Access (SC-FDMA) for the uplink. The allocation of radio resources to mobile terminals, referred to as User Equipments (UEs), for both downlink and uplink is performed by a scheduler located in the access node of the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), the E-UTRAN Node B, commonly abbreviated as eNodeB. The Resource allocation is performed adaptively using fast scheduling, taking into account the instantaneous traffic pattern and radio propagation characteristics for each UE.
In LTE, all packets are delivered using the Internet Protocol (IP). This means that also traditionally circuit switched services, such as voice, will make use of fast scheduling. Since the TTI used in LTE is much shorter than in other wireless technologies, like Global System for Mobile Communications (GSM) and Wideband Code Division Multiple Access (WCDMA), the energy received at the eNodeB will not be sufficient for transmitting a Voice over IP (VoIP) packet, unless the transmission power at the UE is increased. The current solution, which is standardized by the 3GPP, is to segment the VoIP packet into small pieces, transport blocks, and to retransmit the same packet in four consecutive subframes, TTIs, before the feedback of earlier transmissions is received and processed. The soft combining gain is achieved by combining the consecutive transmissions. The Round Trip Time (RTT) for TTI bundling is 16 ms, according to 3GPP specifications. It is further specified that four HARQ processes are used for TTI bundling transmissions, as compared to eight HARQ processes for normal transmissions. When a UE receives an uplink grant, it will invoke the same HARQ process for the consecutive four subframes.
One of the benefits of TTI bundling is a lower overhead due to reduced segmentation and an reduced signaling of lower-layer headers, such as Radio Link Control (RLC) and Medium Access Control (MAC) headers. In addition to that, Layer 1 and Layer 2 messaging is reduced since fewer grants are needed to transmit the same amount of Layer 2 bits. However, since it is not favorable to let UEs which do not segment their VoIP packets use TTI bundling, because of the increased usage of radio resources, a mixture of UEs using TTI bundling and UEs not using TTI bundling within the same cell is expected in realistic scenarios.
In order to switch from normal transmission mode to TTI bundling transmission mode, or vice versa, the eNodeB sends a Radio Resource Control (RRC) Reconfiguration Request signal to the UE, ordering the UE to toggle its TTI bundling mode. In response to the request, the UE toggles its TTI bundling mode and transmits an RRC Reconfiguration Complete signal to the eNodeB. During the time interval between transmitting the request to the UE and receiving the confirmation from the UE, the eNodeB has no knowledge about the current TTI bundling mode of the UE. The UE behavior during the switching period, i.e., between receiving the request from the eNodeB and transmitting a confirmation to the eNodeB, is not well specified. This uncertainty may give rise to problems due to differences between normal transmissions and TTI bundling transmissions, which are related to allocating Physical Uplink Shared Channel (PUSCH) resources, transmitting Hybrid Automatic Repeat Request (HARQ) feedback information, and the mapping of HARQ processes in the eNodeB and the UE, respectively.
In particular, problems may arise if the UE transmits data during an ongoing switching of TTI bundling, i.e., between receiving the RRC Reconfiguration Request and transmitting the RRC Reconfiguration Complete. Since the UE is required to transmit the RRC Reconfiguration Complete using the configuration after the reconfiguration procedure, while data not related to the reconfiguration procedure is transmitted according to the configuration prior to the reconfiguration procedure, the eNodeB has no means of knowing whether data which is received during an ongoing switching procedure was transmitted using TTI bundling or not.
A known solution to overcome this uncertainty is to perform an intra-cell handover at the same time as switching TTI bundling. For this purpose, a handover command is included in the RRC Reconfiguration Request signal. When the UE receives the request, it will start a handover procedure to its own cell while simultaneously toggling its TTI bundling mode. More specifically, the UE sends Random Access (RA) preambles, and the eNodeB schedules an RA Msg3 grant in the RA Msg2, in response to which the UE sends RA Msg3. When the eNodeB receives RA Msg3, it knows that the UE has successfully toggled its TTI bundling mode, and it can schedule the UE according to the new TTI transmission mode. However, while this solution provides a robust way to avoid the uncertainty associated with switching TTI bundling, it gives rise to extensive RA messaging, thereby reducing the overall RA success rate of the system. This, in turn, may result in an increased interference caused by UEs increasing their transmit power in response to failed RA attempts.